Pressure sensors are often included in systems that measure the levels of liquids in storage tanks. These sensors are common in water treatment plants, water supply systems and so forth. One such system is described in U.S. Pat. No. 4,380,933 to Irvin. A mercury manometer is connected along with a pump to a bubbler tube in the storage tank. The pump forces air into the bubbler tube to force liquid from the tube. When all the liquid is forced from the tube, the back pressure in the tube is proportional to the height of the displaced liquid. This pressure is sensed by the manometer, which can be calibrated in terms of the level of liquid in the storage tank.
Mercury is a hazardous substance. Accordingly, the Irvin system contains special filters and reservoirs to prevent the mercury from leaking out of the system. This is particularly important if the system is used to control the water level in the storage tank of a water supply system. However, even with these filters and reservoirs, the mercury may still leak. For example, a well of the manometer may crack and mercury may then flow through the crack, to contaminate the environment or nearby workers. Thus, it is desirable to utilize some substance other than mercury as a sensing element in such a system.
The Irvin system includes various electrical contacts positioned along the manometer to produce control signals. When the mercury in the manometer reaches a contact it immerses, and thus, closes an electrical circuit to produce an associated control signal. This signal turns on a pump that forces liquid from the tank. As the level of liquid in the tank falls, the mercury level in the manometer also falls, until the mercury no longer covers the contact. The circuit thus opens and the pump shuts off.
This feature of the Irvin control system works well for storage systems that do not require precise control of the level of liquid in the storage tank. This system produces various control signals that correspond to specific levels of mercury, and thus, to specific levels of liquid in the tank. Accordingly, the system does not provide continuously varying control signals. Further, each control signal corresponds with the position of a fixed contact. Thus the system is impracticable for use in situations where one might want to change the liquid level corresponding to a particular control signal. For more precise control of the liquid level, what is needed is a system that produces a control signal that responds essentially continuously to changes in the level of liquid in the storage tank.